Sweep signal sources are well known in the art for a variety of test and measurement purposes. Typically, the frequency of a source is swept continuously or in steps between preselected start and stop frequencies. State of the art sweep signal sources operate over frequency ranges on the order of 10 megahertz to 40 gigahertz. Phase locked loop frequency synthesizers are used to provide highly accurate, stable output frequencies. Typical features include continuous or stepped sweeps, selectable start and stop frequencies, and selectable sweep times.
A frequent use of sweep signal sources is in network analyzer systems. A vector network analyzer system contains several elements. A first element is the signal source to provide a stimulus to a device under test (DUT). A second element is a signal separation network to route the stimulus to the DUT and to provide a means for sampling the energy that is reflected from or transmitted through the DUT. Also, energy from the signal that is incident upon the DUT is sampled in order to provide a reference for relative measurements. A third element is a tuned receiver to convert the resulting signals to intermediate frequencies for further processing. The magnitude and phase relationships of the original signals must be maintained through the frequency conversion to intermediate frequency to provide usable measurements. A fourth element is a detector to detect the magnitude and phase characteristics of the intermediate frequency signals, and a fifth element is a display on which to present the measurement results.
In a network analyzer system, it is necessary to synchronize the operations of the receiver to those of the source. The receiver and signal processing portions of the system take data or measurements at a number of frequencies during a sweep. The data must be precisely correlated to the frequencies at which it was taken in order to provide accurate measurements. In addition, network analyzer systems often employ markers to indicate selected frequencies on a display. In order to ensure that the markers appear at the selected frequencies, the receiver must determine when the source sweeps through that frequency. Depending on the selected start and stop frequencies, the signal source may change bands during a sweep by activating different oscillator and/or frequency multiplier configurations. When bands are changed, the sweep is temporarily stopped. The receiver must be notified when a stop sweep occurs in order to maintain synchronization.
It is customary to generate continuous frequency sweeps by applying a continuously increasing or decreasing ramp voltage to the tuning input of a voltage controlled oscillator. Although the ramp voltage can be initiated by a synchronizing signal, the ramp voltage is subject to errors from a number of sources, including timing capacitor and timing resistor tolerances, reference voltage variations, temperature variations and component aging. Furthermore, different components and different voltages are used to generate the ramp voltage, depending on the selected sweep time. When the ramp voltage is in error, the synchronizing signals have an imprecise time relationship to the ramp voltage. Consequently, operations that are synchronized to such signals are not synchronized to the sweep.
In order to reduce such errors and to more accurately synchronize operations to the frequency sweep, it is known to convert the ramp voltage into a digital pulse train using an analog-to-digital converter. Each time the ramp voltage changes by a predetermined amount, a digital pulse is generated. Thus, the synchronizing signal is generated directly from the ramp voltage. This technique is described in U.S. Pat. No. 4,641,086 issued Feb. 3, 1987 to Barr, IV et al and is implemented in the Model 8340 signal source and Model 8510 receiver, both manufactured and sold by Hewlett Packard Company. While this technique provides satisfactory performance, it is subject to errors in the analog-to digital converter which converts the ramp voltage to a digital pulse train.
Swept synthesizers sometimes generate frequency sweeps using a technique known as fractional-N sweeps. In fractional-N phase locked loops, the divider ratio is changed on the fly during a sweep, and phase errors are corrected using analog phase interpolation. The output is a phase locked analog frequency sweep. Such sweeps are generated without the use of a ramp voltage. Thus, it is highly inaccurate to synchronize the receiver by conversion of a ramp voltage to a pulse train when fractional-N sweeps are employed.
The ramp voltage is applied to an oscillator in order to provide a linear frequency sweep. Even when the ramp voltage is accurate, errors can be introduced by the oscillator and the oscillator drive circuitry, which cause the frequency versus time sweep to differ from the desired sweep. Such errors can include errors in the start frequency and error in the slope of the frequency sweep. A technique for correcting the start frequency is disclosed in U.S. Pat. No. 4,130,808 issued Dec. 19, 1978 to Marzalek. It is known to correct slope errors by stopping the sweep, measuring the slope error and then calculating a slope correction. In the prior art technique, the oscillator frequency tends to drift when the sweep is stopped, thereby, introducing additional errors.
It is a general object of the present invention to provide improved sweep frequency sources.
It is another object of the present invention to provide digitally-synchronized sweep frequency sources.
It is yet another object of the present invention to provide highly accurate sweep sources.
It is a further object of the invention to provide methods and apparatus for correcting the slope of a frequency sweep in a sweep frequency source.
It is a yet another object of the present invention to provide methods and apparatus for measuring the frequency of a sweep frequency source at a predetermined time during a sweep.